The present invention relates to operational transconductance amplifiers (OTAs) and more particularly to OTAs with a high common mode excursion, so-called rail to rail OTAs.
FIG. 1 is a simplified drawing of a conventional rail to rail OTA. This OTA comprises an input stage with two complementary MOS differential amplifiers.
The first differential amplifier comprises two N-channel MOS transistors T1a and T2a, the gates of which are respectively connected to a non-inverting input (+) and to an inverting input (-). The sources of the transistors are interconnected and are loaded by a current source Ia connected to a negative supply rail Vss and drawing a current 2I.sub.0.
The second differential amplifier comprises two P-channel MOS transistors T1b and T2b, the gates of which are respectively connected to inputs (+) and (-). The sources of these transistors are interconnected and are loaded by a current source Ib connected to a positive supply rail Vdd and providing a current 2I.sub.0.
The drains of transistors T1a and T2a are respectively connected to the input of a current mirror M1a and to the input of a current mirror M2a. A current mirror is a two-channel circuit which copies on its second channel, here called output, the current present on a first channel, here called input and designated by an arrow. The outputs of the current mirrors M1a and M2a are respectively connected to the drain of transistor T2b and to the drain of transistor T1b. The drain of transistor T2b is connected to the input of a current mirror M2b, the output of which is connected to the input of a current mirror M3. The drain of transistor T1b is connected to the input of a current mirror M1b. The outputs of the current mirrors M3 and M1b are interconnected and constitute the output S of the amplifier.
The normal operation of the circuit is as follows.
In the absence of a differential signal between the inputs (+) and (-), the current in the drains of each transistor T1a, T2a, T1b, T2b, is equal to a quiescent current I.sub.0 equal to half the current 2I.sub.0 fixed by the current sources Ia and Ib.
When a voltage V1 is applied to the input (.) and a voltage V2 to the input (-), a current I1 flows in the drains of each transistor T1a and T2b and a current I2 flows in the drains of each transistor T2a and T1b; currents I1 and I2 verify the following relations: EQU I1+I2=2I.sub.0, and EQU I1-I2=(V1-V2)g.sub.m (I.sub.0)
where g.sub.m (I.sub.0) designates the transconductance of each transistor for a quiescent drain current equal to I.sub.0.
The drain current I1 of transistor T1a is copied to the output of current mirror M1a and is added to the drain current I1 of transistor T2b, the input of current mirror M2b thus receives a current 2I1. Similarly, the drain current I2 of transistor T2a is copied to the output of mirror M2a and is added to the drain current I2 of transistor T1b, mirror M1b thus receives at its input a current 2I2. Thus, mirror M3, the input of which receives the output current 2I1 from mirror M2b, provides at the output S a current 2I1 and mirror M1b draws from output S a current 2I2. Therefore, the output S of the OTA provides a current Is such that: EQU Is=2(I1-I2)=2(V1-V2)g.sub.m (I.sub.0).
The transconductance of the OTA is therefore equal to 2g.sub.m (I.sub.0).
When one of the currents I1 or I2 is null, the other current is equal to the current provided by sources Ia and Ib. Then, the output current Is is maximum and is here 4I.sub.0. This current determines the maximum charge speed of spurious capacitances present in the OTA, and of a capacitance that may be present at the output. This charge speed is usually known as "slew-rate".
In addition to the normal operating range described above, this amplifier comprises two exceptional operating ranges:
if voltages V1 and V2 go too close to the supply voltage Vdd, transistors T1b and T2b are no longer biased and become inactive, their drain currents cancel, and the current mirrors M1b and M2b respectively only receive at their inputs current I2 from mirror M2a and current I1 from mirror M1a; the OTA then provides at its output a current Is such that: EQU Is=I1-I2=(V1-V2)g.sub.m (I.sub.0);
if voltages V1 and V2 go too close to the supply voltage Vss, transistors T1a and T2a become inactive and their drain currents cancel. The mirrors M1b and M2b respectively only receive at their inputs current I2 from transistor T1b and current I1 from transistor T2b; the output current of the OTA is in this case also equal to I1-I2.
Over both these operating ranges, the transconductance of the OTA is equal to g.sub.m (I.sub.0) and the maximum output current is 2I.sub.0.
Therefore, a drawback of such an OTA is that its transconductance is halved as the OTA passes from the normal operating range to one of the exceptional operating ranges. Thus, if a constant transconductance is desired, the common mode of the amplifier is limited to the normal operating range.